1. Field of the Invention
The present invention relates to displays, and more particularly to a driver for an Organic Light-Emitting Diode (OLED) passive-matrix display.
2. Description of the Related Art
Liquid crystal displays (LCDs) are the most common type of flat-panel display used today. One drawback, however, to LCDs is that they require a separate light source, typically a fluorescent backlight, to illuminate the panel. In fact, the LCD's brightness depends solely on its backlight and it is this backlight that limits the life of the LCD.
Because of these drawbacks, OLED displays are gaining in popularity. OLED displays are self-luminous and, therefore, do not require a separate backlight. Passive-matrix OLED displays have a simple structure and are well suited for low-cost and low-information-content applications, such as alphanumeric displays. Active-matrix OLEDs have an integrated electronic backplane that enables high-resolution, high-information-content applications, including videos and graphics. In any event, the OLED displays are very thin, compact displays with wide viewing angles (up to 180 degrees), fast response, high resolution, and good display qualities.
The basic OLED cell includes a stack of thin organic layers sandwiched between an anode and a metallic cathode. The organic layers generally include a hole-injection layer, a hole-transport layer, an emissive layer, and an electron-transport layer. The emissive layer is primarily responsible for the light generation or electroluminescence. Specifically, when an appropriate voltage is applied to the cell, the injected positive and negative charges recombine in the emissive layer to produce light. The structure of the organic layers, of the anode and cathode is designed to maximize the recombination process in the emissive layer, thereby maximizing the light output from the OLED display.
The light output or brightness of an OLED display is directly proportional to current flow. Additionally, the impedance of the OLEDs drops exponentially with an increasing forward voltage (VF). Thus, as impedance drops, light output increases rapidly and there is virtually no delay between the generation of current flow and the generation of light output.
One problem with OLED displays is the variation of the current-voltage (I-V) characteristics over time, which causes degradation of the luminance efficiency and pixel-to-pixel luminance uniformity. Several factors contribute to this variation in the I-V characteristics including operating temperature, external light (e.g., sunlight), pixel position on the display, etc. The driving method also affects the I-V characteristics. For example, in an OLED passive-matrix display, one method used is called multiplexing line address (MLA), wherein the average current needed to bias the OLED is multiplied by the duty cycle of the row to compute an equivalent multiplexing current, which may be 50 to 200 times the average bias current (1 μA to 1 mA from dim to bright) depending on the number of rows and the efficiency of material. Such high currents cause excess voltage drops on the OLEDs that results in wasted power consumption.
International application WO 03/107313A2 to Cambridge Display Technology Limited discloses a technique to reduce power consumption in an active-matrix display by using current and voltage sensors and by controlling an adjustable power supply that adjusts the voltage in response to the sensed voltage. However, this application only discloses indirectly measuring voltage and current used by the display pixels, which is less desirable. Additionally, there is no well-defined technique disclosed for efficient power-up of the OLED display. That is, when the display is first powered on, the pixels are off and the required voltage needed by the OLED display is not well defined.
Thus, there is a need for a display that can efficiently bring the OLEDs through a power-up mode and allow for adjustment of the power levels supplied to the OLEDs after the power-up mode has been completed.